1. Field of the Invention
This invention relates to a method and apparatus for electroplating metals onto a substrate, for example, fine features used in the electronics field, and, in particular, to improving the uniformity of electroplating feature thickness across regions of different feature density.
2. Description of Related Art
The electrical performance of a multi-chip module (MCM) is strongly influenced by the thickness distribution of interconnect metal, i.e., the thickness of deposited metal forming the particular interconnect feature. Circuit pattern densities are not always uniformly distributed on a carrier surface. In some areas, the patterns can be very dense, for example where the wires or other features are relatively closely spaced, while in other areas, the patterns can be very isolated, where the wires or features are spaced relatively far apart. Using prior art plating tools and processes, the plated metal thickness can vary significantly. The resultant thin film interconnect structure of nonuniform thickness can severely impact the electrical performance and the production yields due to high standard deviation of parametric measurements. These may be found in other electronics applications, for example, printed circuit boards and magnetic recording heads.
Paddle cells have been used which employ separate power supplies to impress a current between an anode and the cathode comprising the substrate (workpiece) to be plated, and a current between the anode and a secondary cathode, or thief ring, which surrounds the substrate. Traditionally, plating in a paddle cell is controlled only by adjusting substrate and thief currents. There is little control on the local currents on a substrate. Another method to achieve uniformity includes creating dummy pads in the isolated areas. However, it may generate more electrical performance issue. Another approach was employed in Kaja et al. U.S. Ser. No. 09/699,909, filed on Oct. 30, 2000, which disclosed the use of a woven metallic mesh of uniform spacing placed over the substrate and electrically connected to the thief plate. While the Kaja et al. method worked well in its intended use to plate only very few lines on a substrate, it did not to solve the thickness uniformity problem for areas having different feature density.